Biomedical Engineering Reference
In-Depth Information
the early embryo could also give rise to immortal pluripotent stem cells, termed
embryonic stem (ES) cells. Remarkably, single EC cells introduced into mice can
give rise to teratomas containing multiple cells types demonstrating at minimum
that they have multipotent potential (Kleinsmith and Pierce, JR. 1964 ). In fact, EC
cells represent the first pluripotent stem cell to be described. If EC cells fail to dif-
ferentiate they give rise to malignant teratocarcinomas comprised solely of EC
cells. Importantly, these studies of TGCTs determined that cells committed to
entering the germline, and which might be considered very specialized cells, were
capable of giving rise to a pluripotent stem cell (Stevens 1967a ). Subsequent stud-
ies demonstrated that when PGCs were cultured in vitro they could give rise to
another type of pluripotent stem cell termed an embryonic germ (EG) cells (Matsui
et al. 1992 ; Resnick et al. 1992 ). Previous studies had shown that PGCs could be
isolated from the embryo and cultured on feeder cells, but that the isolated cells
proliferated and differentiated in culture in a manner that mirrored their normal
pattern of differentiation in vivo (Donovan et al. 1986 ). In other words, their growth
in culture seemed to follow the same timing, or developmental clock, as their devel-
opment in vivo . The feeder cells on which the PGCs are grown are known to
express many growth factors including, importantly, a ligand for the C-Kit receptor
termed kit ligand (KL) that is essential for PGC survival (Dolci et al. 1991 ; Godin
et al. 1991 ; Matsui et al. 1991 ). Furthermore, they express leukemia inhibitory fac-
tor (LIF), which can act together with KL to stimulate PGC proliferation (Cheng
et al. 1994 ). But when fibroblast growth factor-2 (FGF2 or basic FGF) is added to
the cultures, the PGCs proliferate for longer than they would normally and eventu-
ally form a population of cells that appear to be immortal (Matsui et al. 1992 ;
Resnick et al. 1992 ). While PGCs normally grow for a short period of time in cul-
ture, PGCs grown in KL, LIF, and FGF2 can be subcultured and expanded indefi-
nitely. In addition to being immortal these cells also are pluripotent. When they are
introduced into blastocyst-stage embryos they incorporate into the embryo proper
and give rise to chimeras containing donor cells that have contributed to the somatic
and germ cell lineages. Importantly, chimeras derived from these cells transmit
donor-derived DNA through the germline (Matsui et al. 1992 ; Stewart et al. 1994 ).
To distinguish these cells from other pluripotent stem cells, such as EC cells and
ES cells derived from the ICM of the pre-implantation embryo, as well as to distin-
guish them from PGCs, they have been termed EG cells (Resnick et al. 1992 ).
Therefore, both in vivo and in vitro , PGCs can give rise to immortal pluripotent
stem cells. The exact relationship between EC cells and EG cells is uncertain but it
is interesting to speculate that some of the same mechanisms that cause PGCs to
give rise to EC cells in vivo might also be involved in the development of EG cells
in vitro and some experimental data supports this idea (see below). One important
clue as to the mechanism of EG cell derivation comes from analyses of the timing
of when EG cells could be made from mouse embryonic PGCs. It was noted that
after 12.5 dpc it was difficult, if not impossible, to derive EG cells (Matsui et al.
1992 ; Resnick et al. 1992 ). Many years earlier it had been noted that it was not
possible to derive experimental TGCTs from PGCs after this time (Stevens 1966 ).
Together these data suggest that some aspect of PGC differentiation that occurs at
Search WWH ::




Custom Search